Co-crystals of tramadol and coxibs

ABSTRACT

The present invention relates to co-crystals of tramadol and co-crystal formers selected from NSAIDs/coxibs, processes for preparation of the same and their uses as medicaments or in pharmaceutical formulations, more particularly for the treatment of pain.

The present invention relates to co-crystals of tramadol and NSAIDs—likecoxibs—, processes for preparation of the same and their uses asmedicaments or in pharmaceutical formulations, more particularly for thetreatment of pain.

Pain is a complex response that has been functionally categorized intosensory, autonomic, motor, and affective components. The sensory aspectincludes information about stimulus location and intensity while theadaptive component may be considered to be the activation of endogenouspain modulation and motor planning for escape responses. The affectivecomponent appears to include evaluation of pain unpleasantness andstimulus threat as well as negative emotions triggered by memory andcontext of the painful stimulus.

In general, pain conditions can be divided into chronic and acute.Chronic pain includes neuropathic pain and chronic inflammatory pain,for example arthritis, or pain of unknown origin, as fibromyalgia. Acutepain usually follows non-neural tissue injury, for example tissue damagefrom surgery or inflammation, or migraine.

There are many drugs that are known to be useful in the treatment ormanagement of pain. Opioids are frequently used as analgesics in pain.Derivatives of morphine are indicated for the treatment of moderate toacute pain in human. The analgesic effect is obtained through theiraction on morphinic receptors, preferably the p-receptors. Among thesederivatives of morphine, may be mentioned morphine, codeine, pethidine,dextropropoxyphenemethadone, lenefopan and others.

One of the morphinic derivatives that has shown very good results whenorally administrated, and which is extensively marketed, is Tramadol,also available as a physiologically acceptable salt, particularly as achlorohydrate. Tramadol, whose chemical name is2-(dimethylaminomethyl)-1-(3-methoxyphenyl)cyclohexanol, has thefollowing formula:

This structure shows two different chiral centers and thus may exist indifferent diastereoisomers among which the tramadol is thecis-diastereisomer: (1R, 2R), or (1S, 2S), both also known as(+)-tramadol and (−)-tramadol and both of which contribute in differentways to its activity.

From the art it appears that this compound is neither fully opioid-like,nor non-opioid-like. Some studies have demonstrate that tramadol is anopioid agonist, whereas clinical experience indicates that it lacks manyof the typical side effects of opioids agonist, for example respiratorydepression, constipation or tolerance.

Due to their drawbacks, opioids cannot always be given repeatedly or athigher doses as analgesics to treat pain. The side effects of opioidsare known in the art including e.g. J. Jaffe in “Goodman and Gilman's,The Pharmacological Basis of Therapeutics”, 8^(th) edition; Gilman etal.; Pergamon Press, New York, 1990, Chapter 22, pages 522-573.

Consequently it has been proposed to combine opioids with other drugsthat are not opioid analgesic agents, in order to lower the amount ofopioids needed to produce an equivalent degree of analgesia. Among thesecombinations, the association of tramadol with nonsteroidalanti-inflammatory drugs (NSAIDs) has been reported to be of particularinterest (EP-0 546 676).

Thus it was the objective of the current invention to provide new meansof improving the properties of tramadol, especially in regard to thetreatment of pain, by providing new drugable forms of tramadol.

Especially desirable improvements/advantages of the new drugable formwould include:

-   -   improvement of physicochemical properties in order to facilitate        the formulation, the manufacture, or to enhance the absorption        and/or the bioavailability:

-    thus    -   being more active when compared to tramadol base or        hydrochloride salt; or    -   providing a form of tramadol with a further active agent having        a beneficial pharmacological effect in itself, thus allowing for        a highly efficient dose/weight relation of the final active        principle or even    -   allowing the use of a lower therapeutic dose of either tramadol        and the further active agent, an NSAID—the coxib—, or of both;    -   having a synergistic effect through the combination of tramadol        and the further active agent, an NSAID—the coxib—, in the same        new drugable form; or

-    further    -   having the bitter taste of tramadol removed or ameliorated;    -   being easily obtainable, easy to manufacture or    -   allowing more flexibility in formulating, or facilitating its        formulation,    -   being highly soluble, thus allowing better dissolution rates,        especially if dissolving in an aqueous physiological        surrounding, or    -   improving stability of the co-crystal in comparison to the        physical mixture of Tramadol/Active Agent (an NSAID—the coxib—)        at the same ratio;    -   allowing new routes of administration;

-    also    -   allowing—if necessary—to combine tramadol with a chemically        usually non-compatible active agent in the same formulation or        even in immediate contact, without having to isolate tramadol;

-    or finally    -   minimizing/reducing the side effects, especially the severe side        effects, assigned to tramadol.

Other desirable improvements/advantages of the new drugable form wouldinclude being active in diseases or symptoms being or related to painand its subtypes, especially those in which current treatment isinsufficient like sciatica or frozen shoulder or pain related to centralsensitization (central pain syndrome).

Most desirably the new drugable forms should combine more than one, mostof these advantages.

This objective was achieved by providing new co-crystals of tramadol. Itwas found that tramadol was able to form co-crystals with NSAIDs—likecoxib—, especially with celecoxib. These co-crystals show improvedproperties if compared to tramadol alone, and also good analgesicactivity. The co-crystals thus obtained have a specific stoichiometry.Under the proper circumstance this is also another advantage of thesenew solid drugable forms possibly achieving some modulation of thepharmacological effects. While APIs (Active Pharmaceutical Ingredients)like tramadol in general have been recognized to form crystallinepolymorphs, solvates, hydrates and amorphous forms for a number ofyears, there is little knowledge about which APIs will form co-crystals.Co-crystals are a specific type of crystalline form which provide a newavenue to modulate the API form and thus to modulate API properties.Co-crystals contain an API and at least one other component whichcrystallize together. Selection of the other component helps determinewhether a co-crystal will form and what properties the co-crystal willhave. Just as a polymorph, solvate, hydrate or amorphous form of an APIcan modulate stability, solubility, and hygroscopicity, a co-crystal canmodulate those same properties.

Thus the main object of the present invention is a co-crystal comprisingtramadol either as a free base or as a physiologically acceptable saltand at least one NSAID/coxib.

The coxibs are highly interesting NSAIDs for being the co-crystal formerwith tramadol. They are selective COX-2 inhibitors. The most importantof these is the marketed drug celecoxib. Its chemical name is4-[5-(4-methylphenyl)-3-(trifluoromethyl)-pyrazol-1-yl]benzenesulfonamide.It has an empirical formula of C₁₇H₁₄F₃N₃O₂S.

NSAIDs like the coxibs have analgesic activity in a number of painsymptoms. The basis of their activity is inhibition of cyclooxygenase(COX), one of the two activities of prostaglandine endoperoxide synthase(PGHS). It is a key enzyme in the prostaglandin pathway.

“Drugable form (of tramadol)” as used herein is defined as any form(salt, amorphous crystal, solution, dispersion, mixture etc,) thattramadol might take which still can be formulated into a pharmaceuticalformulation usable as a medicament to treat a disease or a symptom,especially pain.

“Co-Crystal” as used herein is defined as a crystalline materialcomprising two or more compounds at ambient temperature (20 to 25° C.,preferably 20° C.), of which at least two are held together by weakinteraction, wherein at least one of the compounds is a co-crystalformer. Weak interaction is being defined as an interaction which isneither ionic nor covalent and includes for example: hydrogen bonds, vander Waals forces, and π-π interactions. Solvates of tramadol that do notfurther comprise a co-crystal former are not co-crystals according tothe present invention. The co-crystals may however, include one or moresolvate molecules in the crystalline lattice. Just for the sake ofclarity the distinction between crystalline salt and a co-crystal has tobe stressed here. An API bound to another compound forming a salt bymeans of ionic interaction can be considered as one “compound” accordingto the invention, but it cannot be considered as two compounds byitself.

In scientific literature there currently is some discussion on theproper use of the word co-crystal (see for example Desiraju, Cryst. Eng.Comm., 2003, 5(82), 466-467 and Dunitz, Cryst. Eng. Comm., 2003, 5(91),506). A recent article by Zawarotko (Zwarotko, Crystal Growth & Design,Vol. 7, No. 1, 2007, 4-9) gives a definition of co-crystal which is inline with the definition given above and thus also is a definition of“co-crystal” according to this invention. According to this article “aco-crystal is a multiple component crystal in which all components aresolid under ambient conditions when in their pure form. These componentsconsist of a target molecule or ion and a molecular co-crystalformer(s); when in a co-crystal, they coexist at a molecular levelwithin a single crystal”.

“Co-crystal former” as used herein is defined as a molecule being anactive agent selected from NSAIDs/Coxibs, and with which tramadol isable to form co-crystals.

“Active agents” are APIs which show a pharmaceutical effect and thus canbe identified as being pharmaceutically active. In a more narrow sensethis definition is encompassing all APIs being marketed or underclinical trial for the treatment of diseases. “Active agents withanalgesic activity” are APIs (Active Pharmaceutical Ingredients) whichshow efficacy in well-known animal models of pain and thus can beidentified as being analgesics. In a more narrow sense this definitionis encompassing all APIs being marketed or under clinical trial for alabelling including an indication falling under the definition of pain,including also migraine. These indications might include acute pain,chronic pain, neuropathic pain, hyperalgesia, allodynia or cancer pain,including diabetic neuropathy or diabetic peripheral neuropathy,osteoarthritis or fibromyalgia and all their subforms. Examples of“active agents with analgesic activity” include an NSAID like celecoxibor tramadol and its N-desmethyl-metabolite.

“Pain” is defined by the International Association for the Study of Pain(IASP) as “an unpleasant sensory and emotional experience associatedwith actual or potential tissue damage, or described in terms of suchdamage (IASP, Classification of chronic pain, 2^(nd) Edition, IASP Press(2002), 210). Even though pain is always subjective its causes orsyndromes can be classified. One classification to denominate subtypesof pain would be to divide the general pain syndrome into the subtypesof acute and chronic pain or—according to the pain intensity—into mild,moderate and severe pain. In other definitions the general pain syndromeis also divided into “nociceptive” (caused by activation ofnociceptors), “neuropathic” (caused by damage to or malfunction of thenervous system) and pain related to central sensitization (central painsyndrome).

According to the IASP “allodynia” is defined as “a pain due to astimulus which does not normally provoke pain” (IASP, Classification ofchronic pain, 2^(nd) Edition, IASP Press (2002), 210). Even though thesymptoms of allodynia are most likely associated as symptoms ofneuropathic pain this is not necessarily the case so that there aresymptoms of allodynia not connected to neuropathic pain though renderingallodynia in some areas broader than neuropathic pain.

The IASP further draws the following difference between “allodynia”,“hyperalgesia” and “hyperpathia” (IASP, Classification of chronic pain,2^(nd) Edition, IASP Press (2002), 212):

Allodynia Lowered threshold Stimulus and response mode differHyperalgesia Increased response Stimulus and response rate are the sameHyperpathia Raised threshold; Stimulus and response Increased responserate may be the same or different

According to the IASP “neuropathy” is defined as “a primary lesion ordysfunction in the nervous system” (IASP, Classification of chronicpain, 2^(nd) Edition, IASP Press (2002), 211). Neuropathic pain may havecentral or peripheral origin.

“Sciatica” or “sciatic neuritis is defined herein as a set of symptomsincluding pain that derive from irritation of the sciatic nerve or itsroots,

“Frozen shoulder” or “adhesive capsulitis” is defined herein as asymptom wherein the connective tissue surrounding the shoulder joint orthe shoulder capsule itself, is causing chronic pain, becoming inflamedand stiff.

“Ankylosing spondylitis” or “Morbus Bechterew” is a chronic,inflammatory arthritis and autoimmune disease. It mainly affects jointsin the spine and the sacroilium in the pelvis, causing eventual fusionof the spine.

“Pain related to central sensitization”/“central pain syndrome” isdefined within this application as a neurological condition caused bydamage to or dysfunction of the central nervous system (CNS), whichincludes the brain, brainstem, and spinal cord. This syndrome can interalia be caused by stroke, multiple sclerosis, tumors, epilepsy, brain orspinal cord trauma, or Parkinson's disease.

“Nociceptive pain ” is defined as a type of pain caused by activation ofnociceptors. It can be divided into somatic and visceral pain. “Visceralpain” is pain generally originating from the organs, whereas “(deep)somatic pain” originates from ligaments, tendons, bones, blood vessels,fasciae and muscles.

In one embodiment of the co-crystal according to the invention, theNSAID/s/coxib/s is/are chosen in such a way that if compared to eithertramadol alone, or to a mixture of tramadol and the corresponding activeagent/s/coxib:

-   -   the solubility of the co-crystal is increased; and/or    -   the dose response of the co-crystal is increased; and/or    -   the efficacy of the co-crystal is increased; and/or    -   the dissolution of the co-crystal is increased; and/or    -   the bioavailability of the co-crystal is increased; and/or    -   the stability of the co-crystal is increased; and/or    -   the hygroscopicity of the co-crystal is decreased; and/or    -   the form diversity of the co-crystal is decreased; and/or    -   the morphology of the co-crystal is modulated.

“Mixture of tramadol and the corresponding active agent/s” is defined asa mixture of the active agent or agents in question (the NSAID/coxib)with tramadol which is only a physical mixture without any couplingforces between the compounds and thus neither includes salts nor anotherco-crystal.

In a further embodiment the NSAID being a Coxib is selected fromcelecoxib, etoricoxib, lumiracoxib, parecoxib, rofecoxib, valdecoxib,and cimicoxib.

In a further embodiment of the co-crystal according to the invention,the NSAID being a coxib is selected from:

-   -   Celecoxib,    -   Etoricoxib,    -   Lumiracoxib,    -   Parecoxib,    -   Rofecoxib,    -   Valdecoxib, or    -   Cimicoxib.

Another very preferred aspect of the invention relates to a co-crystalaccording to the invention, wherein the NSAID being a coxib is celecoxibor salts thereof.

Another embodiment of the invention relates to a co-crystal according tothe invention, wherein the tramadol is (−)-tramadol or (+)-tramadol orsalt thereof.

Another embodiment of the invention relates to a co-crystal according tothe invention, wherein the tramadol is (rac)-tramadol or salt thereof.

Especially preferred is a pharmaceutical compound comprising tramadoland celecoxib, preferably a pharmaceutical compound comprising(rac)-tramadol⋅HCl and celecoxib.

As illustrated in more detail below tramadol—and especially theracemate—forms co-crystals with celecoxib. Generally co-crystalsobtained have a specific stoichiometry which depends upon the structureof each co-crystal forming NSAID. In this specific case of theco-crystal between (rac)-tramadol and celecoxib being the co-crystalformer the molecular ratio between tramadol and celecoxib is 1 to 1.

The term “salt” is to be understood as meaning any form of tramadol orthe NSAID/coxib according to the invention in which this assumes anionic form or is charged and is coupled with a counter-ion (a cation oranion) or is in solution. By this are also to be understood complexes oftramadol or the NSAID/coxib with other molecules and ions, in particularcomplexes which are complexed via ionic interactions. This also includesphysiologically acceptable salt.

The term “solvate” according to this invention is to be understood asmeaning any form of the tramadol or NSAID/coxib in which the compoundhas attached to it via non-covalent binding another molecule (mostlikely a polar solvent) especially including hydrates and alcoholsolvates, e.g. methanol solvate.

In a further preferred embodiment of the invention, co-crystal accordingto the invention, is selected from

-   -   a co-crystal comprising (rac)-tramadol either as a free base or        as a physiologically acceptable salt and celecoxib;    -   a co-crystal comprising (+)-tramadol either as a free base or as        a physiologically acceptable salt and celecoxib;    -   a co-crystal comprising (−)-tramadol either as a free base or as        a physiologically acceptable salt and celecoxib; or preferably    -   a co-crystal comprising (rac)-tramadol⋅HCl (the hydrochloride        salt of tramadol) and celecoxib.

A highly preferred embodiment of the co-crystals according to theinvention the co-crystal is formed from (rac)-tramadol either as a freebase or as a physiologically acceptable salt and celecoxib andcelecoxib, preferably from (rac)-tramadol⋅HCl and celecoxib.

In a highly preferred embodiment of these selected co-crystals, themolecular ratio between the (rac)-tramadol⋅HCl and celecoxib is 1:1.

In a preferred embodiment of a co-crystal with a molecular ratio between(rac)-tramadol⋅HCl and celecoxib of 1:1 according to the invention, theco-crystal shows a Powder X-Ray Diffraction pattern with peaks [2θ] at7.1, 9.3, 10.2, 10.7, 13.6, 13.9, 14.1, 15.5, 16.1, 16.2, 16.8, 17.5,18.0, 19.0, 19.5, 19.9, 20.5, 21.2, 21.3, 21.4, 21.8, 22.1, 22.6, 22.7,23.6, 24.1, 24.4, 25.2, 26.1, 26.6, 26.8, 27.4, 27.9, 28.1, 29.1, 29.9,30.1, 31.1, 31.3, 31.7, 32.5, 32.8, 34.4, 35.0, 35.8, 36.2 and 37.2 [°].

The 2θ values were obtained using copper radiation (Cu_(Kα1) 1.54060 Å).

In a preferred embodiment of a co-crystal with a molecular ratio between(rac)-tramadol⋅HCl and celecoxib of 1:1 according to the invention, theco-crystal shows a Fourier Transform Infra Red pattern with absorptionbands at 3481.6 (m), 3133.5 (m), 2923.0 (m), 2667.7 (m), 1596.0 (m),1472.4 (m), 1458.0 (m), 1335.1 (m), 1288.7 (m), 1271.8 (m), 1168.7 (s),1237.3 (m), 1168.7 (s), 1122.6 (s), 1100.9 (m), 1042.2 (m), 976.8 (m),844.6 (m), 820.1 (m), 786.5 (m) 625.9 (m) cm⁻¹.

In a preferred embodiment of a co-crystal with a molecular ratio between(rac)-tramadol⋅HCl and celecoxib of 1:1 according to the invention, theco-crystal has an orthorhombic unit cell with the following dimensions:a=11.0323(7) Åb=18.1095(12) Åc=17.3206(12) Å

In a preferred embodiment of a co-crystal with a molecular ratio between(rac)-tramadol⋅HCl and celecoxib of 1:1 according to the invention, theendothermic sharp peak of the co-crystal corresponding to the meltingpoint has an onset at 164° C.

Another embodiment of the present invention relates to a process for theproduction of a co-crystal according to the invention as described abovecomprising the steps of:

-   -   (a) dissolving or suspending an NSAID—like a coxib—in a solvent;        optionally heating the solution or dispersion to a temperature        above ambient temperature and below the boiling point of the        solution or dispersion;    -   (b) dissolving together with, or after, or before step (a)        tramadol either as a free base or as a salt in a solvent,    -   (c) adding the solution of (b) to the solution of (a) and mixing        them;    -   (d) cooling the mixed solution/dispersion of step (c) to ambient        temperature;    -   (e) optionally evaporating part or all of the solvent; and    -   (f) filtering-off the resulting co-crystals.

Another embodiment of the present invention relates to a process for theproduction of a co-crystal according to the invention as described abovecomprising the steps of:

-   -   (a) dissolving or suspending an NSAID—like a coxib—in a solvent;        optionally heating the solution or dispersion to a temperature        above ambient temperature and below the boiling point of the        solution or dispersion;    -   (b) dissolving together with, or after, or before step (a)        tramadol either as a free base or as a salt in a solvent,        optionally combined with step (a) by dissolving tramadol already        together with the NSAID—like a coxib—in step (a)    -   (c) optionally adding the solution of (b) to the solution of (a)        and mixing them;    -   (d) optionally adding a solvent to the solution of (a), (b)        or (c) and mixing them;    -   (e) cooling the mixed solution/dispersion of step (a), (b), (c)        or (d) to ambient temperature or below;    -   (f) optionally evaporating part or all of the solvent; and    -   (g) filtering-off the resulting co-crystals.

“Ambient temperature” is defined here as a temperature between 20 and25° C., preferably being 20° C.

The solvents usable in these processes include water or organicsolvents, preferably solvents selected from acetone, isobutyl acetate,acetonitrile, ethyl acetate, 2-butanol, dimethylcarbonate,chlorobenzene, butylether, diisopropylether, dimethylformamide, ethanol,water, hexane (also cyclohexane), isopropanol, methyl ethyl ketone (alsomethyl isobutyl-ketone), methanol, methyl t-butyl ether, 3-pentanone,toluene and 1,1,1-trichloroethane, most preferably including alcohols,like ethanol. It is preferable—but not necessary—that the solvents insteps (a) and (c) are identical.

The molecular ratio between tramadol and the NSAID—like the coxib—liesbetween 4:1 to 1:4, preferably from 3:1 to 1:3 and more preferably from1:1 to 1:2.

Preferably the tramadol-solution in step (b) has a concentration ofbetween 3M and 0.01 M.

The parts of the co-crystal according to the invention are well-knowndrugs with analgesic properties sometimes used for a long timeworldwide. Due to this a further object of the present invention is amedicament comprising a co-crystal according to the invention.

Thus the invention also concerns a medicament comprising at least oneco-crystal according to the invention as described above and optionallyone or more pharmaceutically acceptable excipients.

The invention also relates to a pharmaceutical composition thatcomprises a therapeutically effective amount of the co-crystal accordingto the invention in a physiologically acceptable medium.

The association of two active principles in the same crystal exhibitsseveral advantages. Being linked, they often behave as a single chemicalentity, thus facilitating the treatments, formulation, dosage etc. Inaddition to that, with both tramadol and the NSAID—like the coxib—beingactive analgesics these co-crystals are highly useful in the treatmentof pain, especially also not losing any activity/weight by the additionof pharmacologically useless counterions as in salts with no API. Inaddition the two active principles are complementing each other in thetreatment especially of pain, but possibly also of various otherdiseases or symptoms. Thus, the co-crystals according to the inventiondo combine a high number of advantages over the state of the art.

Another advantage is that the association of two active principles intoone unique species seems to allow for a betterPharmacokinetic/Pharmacodynamic (PKPD) including also a betterpenetration of the blood-brain barrier, which helps in the treatment ofpain.

In general, in most embodiments in which the co-crystals of tramadol areused (e.g. for the treatment of pain etc.), these co-crystals would beformulated into a convenient pharmaceutical formulation or a medicament.Accordingly, a desirable advantage of a co-crystal of tramadol wouldshow improved pharmaceutical properties and features, especially whencompared to the free base or tramadol hydrochloride. Thus, theco-crystal of tramadol according to the invention should desirably showat least one, preferably more, of the following features:

-   -   to have a very small particle size, e.g. from 300 μm or lower;        or    -   to be and/or remain essentially free of agglomerates; or    -   to be less or not very hygroscopic; or    -   to help in formulating controlled release or immediate release        formulations; or    -   to have a high chemical stability; or

-    if given to a patient    -   to decrease the inter- and intra-subject variability in blood        levels; or    -   to show a good absorption rate (e.g. increases in plasma levels        or AUC); or    -   to show a high maximum plasma concentration (e.g. C_(max)); or    -   to show decreased time to peak drug concentrations in plasma        (t_(max)); or    -   to show changes in half life of the compound (t_(1/2)), in        whichever direction this change is preferably directed.

The medicament or pharmaceutical compositions according to the presentinvention may be in any form suitable for the application to humansand/or animals, preferably humans including infants, children and adultsand can be produced by standard procedures known to those skilled in theart. The medicament of the present invention may for example beadministered parenterally, including intramuscular, intraperitoneal, orintravenous injection, transmucosal or sublingual application; ororally, including administration as tablets, pellets, granules,capsules, lozenges, aqueous or oily solutions, suspensions, emulsions,sprays or as reconstituted dry powdered form with a liquid medium.

Typically, the medicaments according to the present invention maycontain 1-60% by weight of one or more of the co-crystals as definedherein and 40-99% by weight of one or more auxiliary substances(additives/excipients).

The compositions of the present invention may also be administeredtopically or via a suppository.

The daily dosage for humans and animals may vary depending on factorsthat have their basis in the respective species or other factors, suchas age, sex, weight or degree of illness and so forth. The daily dosagefor humans preferably is in the range of 5 to 500 milligrams of tramadolto be administered during one or several intakes per day.

A further aspect of the invention relates to the use of a co-crystalaccording to the invention as described above for the treatment of pain,preferably acute pain, chronic pain, neuropathic pain, hyperalgesia,allodynia or cancer pain, including diabetic neuropathy orosteoarthritis or fibromyalgia. The invention thus also relates to theuse of a co-crystal according to the invention as described above in theproduction of a medicament for the treatment of pain, preferably acutepain, chronic pain, neuropathic pain, hyperalgesia, allodynia or cancerpain, including diabetic neuropathy or osteoarthritis or fibromyalgia. Afurther aspect of the invention relates to the use of a co-crystalaccording to the invention as described above for the treatment of pain,preferably acute pain, chronic pain, neuropathic pain, severe tomoderate pain, hyperalgesia, allodynia or cancer pain, includingdiabetic neuropathy, osteoarthritis, fibromyalgia; rheumatoid arthritis,ankylosing spondylitis, frozen shoulder or sciatica. The invention thusalso relates to the use of a co-crystal according to the invention asdescribed above in the production of a medicament for the treatment ofpain, preferably acute pain, chronic pain, neuropathic pain, severe tomoderate pain, hyperalgesia, allodynia or cancer pain, includingdiabetic neuropathy, osteoarthritis, fibromyalgia; rheumatoid arthritis,ankylosing spondylitis, frozen shoulder or sciatica. A further aspect ofthe invention relates to a co-crystal according to the invention asdescribed above for (use in) the treatment of pain, preferably acutepain, chronic pain, neuropathic pain, hyperalgesia, allodynia or cancerpain, including diabetic neuropathy or osteoarthritis or fibromyalgia. Afurther aspect of the invention relates to the co-crystal according tothe invention as described above for (use in) the treatment of pain,preferably acute pain, chronic pain, neuropathic pain, severe tomoderate pain, hyperalgesia, allodynia or cancer pain, includingdiabetic neuropathy, osteoarthritis, fibromyalgia; rheumatoid arthritis,ankylosing spondylitis, frozen shoulder or sciatica. Preferably theseuses are provided for in form of a medicament or a pharmaceuticalcomposition according to the invention as described above.

A further aspect of the invention relates to a co-crystal according tothe invention as described above for (use in) the treatment of pain, orpreferably acute pain, chronic pain (acute and chronic pain),neuropathic pain, nociceptive pain (visceral and/or somatic pain), mildand severe to moderate pain, hyperalgesia, pain related to centralsensitization, allodynia or cancer pain, including diabetic neuropathyor diabetic peripheral neuropathy and osteoarthritis, fibromyalgia;rheumatoid arthritis, ankylosing spondylitis, frozen shoulder orsciatica. A further aspect of the invention relates to the use of aco-crystal according to the invention as described above for thetreatment of pain, preferably acute pain, or preferably acute pain,chronic pain (acute and chronic pain), neuropathic pain, nociceptivepain (visceral and/or somatic pain), mild and severe to moderate pain,hyperalgesia, pain related to central sensitization, allodynia or cancerpain, including diabetic neuropathy or diabetic peripheral neuropathyand osteoarthritis, fibromyalgia; rheumatoid arthritis, ankylosingspondylitis, frozen shoulder or sciatica. The invention thus alsorelates to the use of a co-crystal according to the invention asdescribed above in the production of a medicament for the treatment ofpain, preferably acute pain, chronic pain (acute and chronic pain),neuropathic pain, nociceptive pain (visceral and/or somatic pain), mildand severe to moderate pain, hyperalgesia, pain related to centralsensitization, allodynia or cancer pain, including diabetic neuropathyor diabetic peripheral neuropathy and osteoarthritis, fibromyalgia;rheumatoid arthritis, ankylosing spondylitis, frozen shoulder orsciatica. Preferably these uses are provided for in form of a medicamentor a pharmaceutical composition according to the invention as describedabove.

The uses of a co-crystal according to the invention (described above) orthe respective method of treatment (described below) is preferablyrelated to pain, including nociceptive pain (which includes somatic andvisceral pain). These preferred embodiments of the invention may also berelated to neuropathic pain and/or to pain related to centralsensitization (the so-called “central pain syndrome”).

The uses of a co-crystal according to the invention (described above) orthe respective method of treatment (described below) may preferably alsobe related to acute and chronic pain.

The uses of a co-crystal according to the invention (described above) orthe respective method of treatment (described below) may preferably alsobe related to mild, to moderate and to severe pain.

Another object of the current invention is a method of treatment ofpain, preferably acute pain, chronic pain, neuropathic pain,hyperalgesia, allodynia or cancer pain, including diabetic neuropathy orosteoarthritis or fibromyalgia, by providing to a patient in needthereof a sufficient amount of a co-crystal according to the inventionas described above. Another object of the current invention is a methodof treatment of pain, preferably acute pain, chronic pain (acute andchronic pain), neuropathic pain, nociceptive pain (visceral and/orsomatic pain), mild and severe to moderate pain, hyperalgesia, painrelated to central sensitization, allodynia or cancer pain, includingdiabetic neuropathy or diabetic peripheral neuropathy andosteoarthritis, fibromyalgia; rheumatoid arthritis, ankylosingspondylitis, frozen shoulder or sciatica, by providing to a patient inneed thereof a sufficient amount of a co-crystal according to theinvention as described above. Preferably the co-crystal according to theinvention is provided in physiologically suitable form like e.g. in formof a medicament or a pharmaceutical composition according to theinvention as described above.

The present invention is illustrated below with the help of thefollowing figures and examples. These illustrations are given solely byway of example and do not limit the invention.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1: Powder X-Ray diffraction pattern of the(rac)-tramadol⋅HCl-celecoxib (1:1) co-crystal.

FIG. 2: DSC analysis of the (rac)-tramadol⋅HCl-celecoxib (1:1)co-crystal.

FIG. 3: TG analysis of the (rac)-tramadol⋅HCl-celecoxib (1:1)co-crystal.

FIG. 4: Structure of the unit cell of the (rac)-tramadol⋅HCl-celecoxib(1:1) co-crystal obtained by SCXRD analysis showing two molecules ofcelecoxib and two molecules of tramadol.

FIG. 5: Showing bioavailability of a co-crystal of(rac)-tramadol⋅HCl-celecoxib (1:1) in dogs compared to celecoxib aloneand to the combination of both API's (the mixture of tramadol andcelecoxib).

FIG. 6: Comparison of the effects of a co-crystal of(rac)-tramadol⋅HCl-celecoxib (1:1), of tramadol and of celecoxib onreversal of incision-induced mechanical allodynia in the incised rathind paw following a single dose (8-10 per group). All data arepresented as means±SEM.

FIG. 7: Isobologram showing anti-allodynic interaction of celecoxib(ED₅₀=3.01 mg/kg) and tramadol (ED₅₀=5.28 mg/kg) on mechanical allodyniain the paw incision postoperative pain model in rats. The oblique linebetween x- and y axis is the theoretical additive line. The point in themiddle of this line is the theoretical additive point calculated fromseparate ED₅₀. Red: the experimental point (co-crystal of(rac)-tramadol⋅HCl-celecoxib (1:1) ED50, molecular weight ratio 1:1.27)lies far below the theoretical ED50 (blue), indicating a significant(P<0.05) synergistic interaction.

FIG. 8: Comparison of the effects of a co-crystal of(rac)-tramadol⋅HCl-celecoxib (1:1), of tramadol and of celecoxib onreversal of incision-induced thermal hyperalgesia in the incised rathind paw following a single dose (8-10 per group). All data arepresented as means±SEM.

FIG. 9: Effects of a co-crystal of (rac)-tramadol⋅HCl-celecoxib (1:1)[the right column in each triplet of bars], of tramadol [thecentral/middle column in each triplet of bars] and of celecoxib [theleft column in each triplet of bars] given intraperitoneally 4.5 h afterinduction by carrageenan (n=8-10 per dose group), on motor behaviour ofmonoarthritic rats measured walking across the CBMS 30 min after drugsadministration.

EXAMPLES Example 1: (rac)-tramadol⋅HCl-celecoxib (1:1) Co-crystal

Processes to Obtain (rac)-tramadol⋅HCl-celecoxib (1:1) Co-crystal:

Example 1a: (Preparation Via Solvent-Assisted Grinding)

A 5 mL stainless steel ball-mill reactor was charged with two 7 mm steelballs, (rac)-tramadol hydrochloride (48 mg, 0.16 mmol), celecoxib (61mg, 0.16 mmol, 1 eq) and 1 drop of methyl isobutyl ketone. The reactorwas agitated at 30 Hz for 45 minutes. Traces of solvent were removed invacuo affording (rac)-tramadol⋅HCl-celecoxib (1:1) co-crystal as a whitesolid (109 mg, quantitative yield).

Example 1b: (Large Scale Via Crystallization)

To a 1 L three necked flask equipped with mechanical stirrer, additionfunnel and cooler containing tramadol⋅HCl (26.54 g, 88.5 mmol) andcelecoxib (33.74 g, 88.5 mmol, 1 eq.), was added 122 mL ethanol. Theresultant suspension was heated to reflux (complete dissolution).Cyclohexane (203 mL) was added slowly to the solution maintaining thereflux (addition time 20 min) and then, the solution was cooled slowlyto room temperature with stirring. The solution was seeded at 55° C.with form obtained in Example 1a and the crystallization started). Themixture was cooled 2 h at 0° C.

The white solid was filtered with a sintered funnel n^(o) 3 and washedwith a solvent mixture at 0-5° C. (1 vol., 60 mL, (0.6:1)EtOH/cyclohexane). After drying 2 days at room temperature under vacuum,(rac)-tramadol⋅HCl-celecoxib (1:1) co-crystal was obtained as a whitesolid (54.6 g, 91% yield).

Characterisation of the Co-crystal:

(rac)-Tramadol⋅HCl-celecoxib (1:1) co-crystal obtained according toexample 1 was fully characterised by ¹H-NMR, FTIR, powder X-Raydiffraction, DSC and TG (see FIGS. 1 to 3).

Powder X-ray Diffraction (PXRD) Pattern of a(rac)-tramadol⋅HCl-celecoxib (1:1) Co-crystal: (see FIG. 1):

PXRD analysis was performed using a Philips X'Pert diffractometer withCu K_(α) radiation in Bragg-Brentano geometry. The system is equippedwith a monodimensional, real time multiple strip detector. Themeasurement parameters were as follows: the range of 2θ was 3° to 40° ata scan rate of 8.8° per minute (see FIG. 1). The peaks expressed inangles 2θ and d-values are described in detail in table 1:

TABLE 1 List of selected peaks obtained by powder X-Ray diffraction of(rac)- tramadol•HCl-celecoxib (1:1) co-crystal. Angle 2θ¹ d-Value (Å)Relative Intensity % 7.06 12.52 29 9.32 9.49 1 10.21 8.67 5 10.69 8.27 213.64 6.49 10 13.86 6.39 14 14.13 6.27 100 15.53 5.71 3 16.10 5.51 616.25 5.45 5 16.85 5.26 44 17.50 5.07 12 18.00 4.93 11 19.05 4.66 3819.48 4.56 11 19.91 4.46 25 20.48 4.34 25 21.18 4.19 11 21.27 4.18 1121.44 4.14 8 21.80 4.08 18 22.14 4.02 3 22.56 3.94 14 22.73 3.91 3223.57 3.78 15 24.11 3.69 13 24.36 3.65 10 25.20 3.53 6 26.12 3.41 1126.58 3.35 2 26.77 3.33 2 27.45 3.25 5 27.94 3.19 4 28.13 3.17 3 29.073.07 9 29.91 2.99 3 30.15 2.96 3 31.11 2.88 3 31.34 2.85 5 31.74 2.82 132.49 2.76 3 32.83 2.73 2 34.42 2.61 1 35.04 2.56 2 35.76 2.51 1 36.242.48 2 37.19 2.42 1 ¹The 2θ values were obtained using cupper radiation(Cu_(Kα) 1.54060 Å)

¹H-NMR Spectrum of a Co-Crystal of (Rac)-Tramadol-HCI and Celecoxib(1:1):

Proton nuclear magnetic resonance analyses were recorded in methanol-d₄in a Varian Mercury 400 spectrometer, equipped with a broadband probeATB 1H/19F/X of 5 mm. Spectra were acquired dissolving 5-10 mg of samplein 0.6 mL of deuterated solvent.

¹H NMR spectrum (in d4-methanol at 400 MHz) δ shows peaks at 7.97-7.90(m, 2H); 7.53-7.46 (m, 2H); 7.30 (t, J=8.0 Hz, 1H); 7.22-7.14 (m, 4H);7.12-7.09 (m, 1H); 7.07 (d, J=7.8 Hz, 1H); 6.90 (s, 1H); 6.83 (dd, J=2.7Hz, J=8.2 Hz, 1H); 3.80 (s, 3H); 2.98 (dd, J=9.0 Hz, J=13.3 Hz, 1H);2.75-2.60 (m, 8H); 2.35 (s, 3H); 2.28-2.18 (m, 1H); 2.00-1.46 (m, 8H)ppm.

FT-IR Spectrum of a Co-crystal of (rac)-tramadol⋅HCl and Celecoxib(1:1):

FTIR spectra were recorded using a Thermo Nicolet Nexus 870 FT-IR,equipped with a beamsplitter KBr system, a 35 mW He—Ne laser as theexcitation source and a DTGS KBr detector. The spectra were acquired in32 scans at a resolution of 4 cm⁻¹.

The sample (KBr pellet) shows a Fourier Transform Infra Red spectrumwith absorption bands at 3481.6 (m), 3133.5 (m), 2923.0 (m), 2667.7 (m),1596.0 (m), 1472.4 (m), 1458.0 (m), 1335.1 (m), 1288.7 (m), 1271.8 (m),1168.7 (s), 1237.3 (m), 1168.7 (s), 1122.6 (s), 1100.9 (m), 1042.2 (m),976.8 (m), 844.6 (m), 820.1 (m), 786.5 (m) 625.9 (m) cm⁻¹.

DSC Analysis of a Co-crystal of (rac)-tramadol⋅HCl and Celecoxib (1:1)(see FIG. 2):

DSC analyses were recorded with a Mettler DSC822^(e). A sample of 1.6230mg was weighed into 40 μL aluminium crucible with a pinhole lid and washeated, under nitrogen (50 mL/min), at 10° C./min from 30 to 200° C.

The novel type of crystal of the present invention is characterized inthat the endothermic sharp peak corresponding to the melting point hasan onset at 164.44° C. (fusion enthalpy—93.56 J/g), measured by DSCanalysis (10° C./min) (see FIG. 9).

TG Analysis of a Co-crystal of (rac)-tramadol⋅HCl and Celecoxib (1:1)(see FIG. 3):

Thermogravimetric analyses were recorded in a thermogravimetric analyzerMettler TGA/SDTA851^(e). A sample of 3.0560 mg was weighed into a 70 μLalumina crucible with a pinhole lid and was heated at 10° C./min from 30to 200° C., under nitrogen (50 mL/min).

The TG analysis of the crystalline form according to the invention showsinsignificant weight loss between 30 and 200° C.

Single Crystal XRD Analysis of a Single Crystal of a Co-crystal of(rac)-tramadol⋅HCl and Celecoxib (1:1) (see FIG. 4):

The crystal structure was determined from single crystal X-raydiffraction data. The colourless prism used (0.33×0.16×0.11 mm) wasobtained from the crystallization of a seeded solution in heptane andIPA of equimolar amounts of (rac)-tramadol hydrochloride and celecoxib.

Analysis was performed at room temperature using a Bruker Smart Apexdiffractometer with graphite monochromated Mo K_(α) radiation equippedwith a CCD detector. Data were collected using phi and omega scans(program used: SMART 5.6). No significant decay of standard intensitieswas observed. Data reduction (Lorentz and polarization corrections) andabsorption correction were applied (program used: SAINT 5.0).

The structure was solved with direct methods and least-squaresrefinement of F_(o) ² against all measured intensities was carried out(program used: SHELXTL-NT 6.1). All non-hydrogen atoms were refined withanisotropic displacement parameters. Crystal data and structurerefinement for (rac)-tramadol-celecoxib (1:1) co-crystal is given in thefollowing table 2.

TABLE 2 Most relevant structural data of the SCXRD analysis of aco-crystal of (rac)-tramadol•HCl celecoxib (1:1) Crystal systemOrthorhombic Space group Pna2₁ a (Å) 11.0323(7) b (Å) 18.1095(12) c (Å)17.3206(12) Volume (Å³) 3460.5(4) Z 4 D calc. (Mg/m³) 1.308 N. of refl.8336 Refl. with I > 2σ(l) 5240 R (I > 2σ(I)) 0.0584

The crystal structure is depicted in FIG. 4 (only half of the unitcellcontents is shown, hydrogen atoms have been omitted for clarity; programused: Mercury 2.2, C. F. Macrae, I. J. Bruno, J. A. Chisholm, P. R.Edgington, P. McCabe, E. Pidcock, L. Rodriguez-Monge, R. Taylor, J. vande Streek and P. A. Wood, J. Appl. Cryst., 41, 2008, 466-470).

Simulation of XRPD diffractogram from single crystal data gives analmost identical diagram to the experimental one presented above.

Example 1c: Determination of the Bioavailability of Co-crystal of(rac)-tramadol⋅HCl-celecoxib (1:1) (Dogs)

The objective was to measure plasma exposure of (rac)-tramadol⋅HCl andcelecoxib in dogs by means of AUC determination of the co-crystal of(rac)-tramadol⋅HCl-celecoxib (1:1) of the present invention, andcomparing it with each active principle of the co-crystal and the fixedcombination of the two active principles.

Bioavailability of (rac)-tramadol⋅HCl-celecoxib co-crystal was comparedto those obtained after administration of (rac)-tramadol⋅HCl pluscelecoxib, combined and separately, to beagle dogs (3 males and 3females) by oral route. Products with an equivalent particle size wereorally administered by means of capsules at a dose level of 10 mg/kg ofco-crystal (as base) and at an equivalent dose level of comparators (4.1mg tramadol/kg, 5.9 mg celecoxib/kg). Blood from the dogs was extractedat the following time points: predose, 15 and 30 min; 1, 1.5, 2, 2.5, 3,3.5, 4, 4.5, 5, 6, 8 and 24 h. The plasma was isolated bycentrifugation, purified by SPE and plasma levels were determined byLC-MS-MS. Pharmacokinetic parameters were calculated usingnon-compartmental pharmacokinetic analysis.

The results showed an increase exposure of celecoxib when the co-crystal(rac)-tramadol⋅HCl-celecoxib was administered compared to celecoxibalone and to the combination of both API's (the mixture of tramadol andcelecoxib) (see FIG. 5).

Example 1d: Effects on Mechanical Allodynia and Thermal Hyperalgesia ina Postoperative Pain Model in Rat

The aim of this study was to evaluate the analgesic efficacy and potencyof co-crystal of (rac)-tramadol⋅HCl-celecoxib (1:1), tramadol andcelecoxib in a rat model of postoperative pain after paw incision.Following plantar incision, rats show decreases in response thresholdsto temperature (thermal hypersensitivity) and graded von Frey filaments(mechanical hypersensitivity) (Brennan et al., Pain 1996, 64, 493).

To assess the reliability of the efficacy and potency of the compoundstested, two different behavioural assays have been used: tactileallodynia using up and down paradigm by von Frey filaments and thermalhyperalgesia using the plantar test assay (Hargreaves et al., Pain 1988,32, 77).

Experimental Design:

Animals

Male, Wistar rats (120-160 g, Harlan, Italy) were housed in aclimate-controlled room for at least 5 days prior to testing. Food andwater were available ad libitum up to test time.

Animal Dosing

The rats were all dosed intraperitoneally with co-crystal of(rac)-tramadol⋅HCl-celecoxib (1:1) or each agent separately, dissolvedin a suspension of 0.5% hydroxypropyl methylcellulose in distilledwater. The dosing volume was 10 ml/kg. The antihyperalgesic orantiallodynic response of the animal was subsequently evaluated 60 minafter drug administration.

Surgery

Rats were anaesthetized with 3% isofluorane for veterinary use,employing an Ohmeda vaporizer and an anaesthesia chamber. Anaesthesiawas kept during the surgical operation by a tube which directs theisoflurane vapours to the animal's snout. Once the rats wereanaesthetised, they were laid down in a prone position and their righthindpaws were cleaned out with alcohol. Then, a 1 cm longitudinalincision was made with a number 23 scalpel, through skin and fascia ofthe plantar aspect of the paw, starting 0.5 cm from the proximal edge ofthe heel and extending toward the toes. Therefore, both superficial(skin) and deep (muscle) tissues and nerves were injured. Finally, theskin of the paw was stitched with a suturing stitch with breaded silk(3.0) and the wound was cleaned out with povidone.

Assessment of Analgesic Activity in Post-Operative Pain in Rats

The drugs were tested 4 hours after the surgery (plantar incision); 60minutes after the administration of the product, two behaviouralendpoints were evaluated: thermal hypersensitivity or hyperalgesia, andmechanical hypersensitivity or allodynia.

Assessment of Thermal Hypersensitivity (Hyperalgesia) in Post-OperativePain in Rats

Hypersensitivity or hyperalgesia was assessed by measurement of aresponse to a thermal stimulus using a Hargreaves apparatus (Ugo Basileplantar test) which selectively elevates the temperature of anindividual paw (Dirig, et al., J Neurosci Methods, 1997, 76, 183).Animals were placed in the methacrylate cages of said apparatus, havinga crystal floor. The acclimation period within the cages was about 10minutes. The thermal stimulus came from a lamp moving below the crystalfloor and which was applied to both paws, with a minimum interval of 1minute between both stimulations in order to avoid learning behaviours.The rat is able to withdraw the paw freely when it feels discomfort(pain) produced by the heat coming from the lamp; then it is switchedoff and the latency time to the withdrawal response is recorded inseconds. In order to avoid hurting the animal's paw, the lamp wasautomatically switched off after 32 seconds. Hyperalgesia is defined asa reduced latency to response compared to the latency of a vehicletreated animal, and the analgesic effect of the test compound is seen asa (partial) restoration of the latency toward normal (Dirig, et al., J.Pharmacol Expt Therap. 1998, 285, 1031).

Assessment of Mechanical Hypersensitivity (Allodynia) in Post-OperativePain in Rats

Mechanical allodynia was tested using von Frey filaments. Animals wereplaced in methacrylate cylinders on an elevated surface, with metallicmesh floor perforated in order to apply the filaments. After anacclimation period of about 30 minutes within the cylinders, bothhindpaws were stimulated (the injured and the non-injured paw, servingthe latter as control), starting with the lowest force filament (0.4 g)and reaching up to a 15 g filament. The animal's response to pain wasmanifested by the withdrawal of the paw as a consequence of the painfulstimulus caused by a filament. The pressure (force in grams) thresholdeliciting the withdrawal of the paw was recorded. The analgesic effectof the test compound is seen as a (partial) restoration of the thresholdtoward normal.

Analysis of Synergistic Effect

The synergistic interaction between tramadol and celecoxib wasdetermined by isobologram analysis as discloses by R. J. Tallarida, etal., Life Sci., 1989, 45, 947. This procedure involves the determinationof the total amount in the mixture that is required to produce aspecified synergistic anti-hyperalgesic effect at the 50% dose level(that is, the ED₅₀ or Zmix) and the corresponding total amount thatwould be expected under simple additivity (ED₅₀ add or Zadd). Where itis established that Zmix<Zadd for a specific fixed-ratio, then thatcomposition has a synergistic anti-hyperalgesic effect. Both ED₅₀ mixand ED₅₀ add values are random variables. ED₅₀ mix is determined fromthe dose-response curve for a specific fixed-ratio of the components;ED₅₀ add is calculated from the ED₅₀ values for the individual drugs.Zmix is then compared to Zadd via a Student's t-test.

Results:

In this study, a dose response of co-crystal of(rac)-tramadol⋅HCl-celecoxib (1:1), tramadol and celecoxib(intraperitoneal route) were obtained. Mechanical allodynia and thermalhypersensitivity were used as behavioural endpoint. All drugs inducedfull efficacy when mechanical allodynia was evaluated.

Results obtained on the effect in mechanical allodynia for co-crystal of(rac)-tramadol⋅HCl-celecoxib (1:1), tramadol and celecoxib expressed asED₅₀ are shown in Table 3 and FIG. 6, whereas FIG. 7 shows anisobologram of anti-allodynic interaction of celecoxib (ED₅₀=3.01 mg/kg)and tramadol (ED₅₀=5.28 mg/kg) on mechanical allodynia in thispost-surgical pain model. FIG. 8 shows results obtained on the effect ofco-crystal of (rac)-tramadol⋅HCl-celecoxib (1:1), tramadol and celecoxibexpressed as ED50 in the incision-induced thermal hyperalgesia in therat hind paw. Co-crystal of (rac)-tramadol⋅HCl-celecoxib (1:1) was morepotent than tramadol and celecoxib.

TABLE 3 ED50 (mg/kg) of each drug tested obtained after sigmoidaladjustment for mechanical allodynia and thermal hyperalgesia in a modelof postoperative pain after paw incision in rats. ED-50 CelecoxibTramadol Co-Crystal Behavioural Mechanical 3.03 5.41 2.04 endpointallodynia Thermal 2.35* 8.3 2.26 hyperalgesia *Emax 47.53%

The isobologram of FIG. 7 is showing anti-allodynic interaction ofcelecoxib (ED₅₀=3.01 mg/kg) and tramadol (ED₅₀=5.28 mg/kg) on mechanicalallodynia in the paw incision postoperative pain model in rats. Theoblique line between x- and y axis is the theoretical additive line. Thepoint in the middle of this line is the theoretical additive pointcalculated from separate ED₅₀. Grey: the experimental point (co-crystalof (rac)-tramadol⋅HCl-celecoxib (1:1) ED₅₀, molecular weight ratio1:1.27) lies far below the theoretical ED₅₀ (black), indicating asignificant (P<0.05) synergistic interaction.

As shown in FIG. 8 tramadol and co-crystal of(rac)-tramadol⋅HCl-celecoxib (1:1) similarly showed full efficacy whenthermal hyperalgesia was used although celecoxib induced only a partialresponse (Emax: 45%). Clearly co-crystal of (rac)-tramadol⋅HCl-celecoxib(1:1) was more potent than tramadol (ED₅₀ Tram: 8.3 mg/kg vs ED₅₀co-crystal of (rac)-tramadol⋅HCl-celecoxib (1:1): 2.26 mg/kg) in thisparameter suggesting a clear synergistic effect. Isobologram analysisfor this behavioural endpoint was not suitable because the ceilingeffect of celecoxib (45%).

Conclusion

Co-crystal of (rac)-tramadol⋅HCl-celecoxib (1:1) administratedintraperitoneal act synergistically to inhibit both mechanical allodyniaand thermal hyperalgesia in the paw incision postoperative pain model

Example 1e: Effects on Mechanical Allodynia and Movement-Evoked Pain inan Acute Monoarthritic Model in Rat

In this study the effects of co-crystal of (rac)-tramadol⋅HCl-celecoxib(1:1), tramadol and celecoxib on mechanical allodynia andmovement-evoked pain were evaluated in an acute monoarthritic rat model.Movement-evoked pain was evaluated by a computerized behavioralmonitoring system (CBMS). The use of this method to assess apain-induced gait adaptation together with the von Frey methodology toevaluate evoked-pain produces a better and more reliable picture of theanimal's pain experience.

The rat carrageenan model utilizes inflammation-associated painfollowing injection of the knee joint (monoarthritic model). The aim ofthis study was to evaluate the analgesic efficacy and potency oftramadol, celecoxib and co-crystal of (rac)-tramadol⋅HCl-celecoxib (1:1)in reducing pain behaviour in monoarthritic rats induced by an injectionof 300 μg of carrageenan into the right knee joint. The CBMS was used toassess gait related changes 5 hours after carrageenan injection and 30min after drugs administration. Gait deficits were observed in differentCBMS parameters grouped in: statics (print area, print length, printwidth), dynamics (stand, swing), and coordination (phase dispersion).Mechanical allodynia was measured with von Frey filaments 15 minutesafter CBMS gait analysis. In this study, the effects of co-crystal of(rac)-tramadol⋅HCl-celecoxib (1:1), tramadol and celecoxib wereevaluated due to their common prescription for moderate to severe painassociated with injury or inflammation in the clinic.

Experimental Design:

Animals

Male, Wistar rats (225-250 g, Charles River Laboratories) were housed ina climate-controlled room. Food and water were available ad libitum upto test time.

Animal Dosing

Rats were all dosed intraperitoneally with co-crystal of(rac)-tramadol⋅HCl-celecoxib (1:1) or each agent tramadol hydrochlorideand celecoxib separately, dissolved in a suspension of 0.5%hydroxypropyl methylcellulose in distilled water. The dosing volume was2 ml/kg. The drug response of the animal was subsequently evaluated 30and 45 min after drug administration (for CBMS and von Freyrespectively).

Knee Joint Monoarthritis Induced by Intraarticular Injection ofCarrageenan.

Joint inflammation was induced by carrageenan (Sigma Chemical, St.Louis, Mo.) injected percutaneously using a 30-gauge needle through theinfrapatellar ligament into the right knee joint cavity (300 μg, 40 μl)with the animal under brief isoflurane (IsoFlo®, Abbott-Esteve,Barcelona, Spain) anaesthesia (3%).

Assessment of Pain-Induced Gait Adaptation with CBMS Method.

Detailed analysis of gait was performed on walking rats using the CBMSmethod. Briefly, light from a fluorescent tube is sent through a glassplate. Light rays are completely reflected internally. As soon asanything, e.g. a rat's paw, is in contact with the glass surface, lightis reflected downwards. It results in a sharp image of a bright pawprint. The whole run is recorded via a camera placed under the glassplate.

In the present study, parameters related to single paws were analyzed:

-   Print Area (expressed in mm²): This parameter describes the total    floor area contacted by the paw during the stance phase.-   Max Contact Area (expressed in mm²): The maximal contact area    describes the paw area contacted at the moment of maximal paw-floor    contact, during stance.-   Print Width (expressed in mm): It is a measure of the width of the    print area.-   Print Length (expressed in mm): It is a measure of the length of the    print area.-   Stand (expressed in s): It is the duration in seconds of contact of    a paw with the glass plate.-   Swing (expressed in s): It is the duration in seconds of no contact    of a paw with the glass plate.-   Swing Speed (expressed in m/s): It is the speed (Distance    Unit/second) of the paw during Swing. This parameter is computed    from stride length and swing duration.-   Duty cycle (%): It expresses Stand as a percentage of Step Cycle.-   Stand Index: It is a measure for the speed at which the paw loses    contact with the glass plate.-   Max Contact at (expressed in seconds): It is the time in seconds    since the start of the run that a paw makes maximum contact with the    glass plate. It can be regarded as the point at which the braking    phase turns into the propulsion phase during Stand.

Assessment of Mechanical Hypersensitivity (Allodynia) in MonoarthriticRats.

Mechanical allodynia was tested using von Frey filaments: Animals wereplaced in methacrylate cylinders on an elevated surface, with metallicmesh floor perforated in order to apply the filaments. After anacclimation period of about 15 minutes within the cylinders, bothhindpaws were stimulated (the injured and the non-injured paw, servingthe latter as control), starting with the lowest force filament (0.4 g)and reaching a 15 g filament. The animal's response to pain wasmanifested by the withdrawal of the paw as a consequence of the painfulstimulus caused by a filament. The pressure (force in grams) thresholdeliciting the withdrawal of the paw was recorded. The analgesic effectof the test compound is seen as a (partial) restoration of the thresholdtoward normal.

Results:

Arthritis caused by injection of carrageenan (CAR) into an ankle jointcaused a change in several parameters describing the way rats walk,indicating an unwillingness to use the injected paw. CAR-induced gaitchanges were inhibited by celecoxib, tramadol and co-crystal of(rac)-tramadol⋅HCl-celecoxib (1:1) (FIG. 9).

The results showed that co-crystal of (rac)-tramadol⋅HCl-celecoxib (1:1)administration produced greater beneficial effects (relative to theindividual administration of tramadol and celecoxib) in variousCBMS-associated parameters including: print area, print length, maximalcontact area, stand index, and phase dispersion.

FIG. 9 shows the effects of a co-crystal of (rac)-tramadol⋅HCl-celecoxib(1:1) [the right column in each triplet of bars], of tramadol [thecentral/middle column in each triplet of bars] and of celecoxib [theleft column in each triplet of bars] given intraperitoneally 4.5 h afterinduction by carrageenan (n=8-10 per dose group), on motor behaviour ofmonoarthritic rats measured walking across the CBMS 30 min after drugsadministration. As outlined above the co-crystal was given at the doseof 20 mg/kg and (rac)-tramadol⋅HCl or celecoxib given alone at the dosecorresponding to which it is present in the co-crystal. Print Area(expressed in mm²) describes the total floor area contacted by the pawduring the stance phase. Max Contact Area describes the paw areacontacted at the moment of maximal paw-floor contact, during stance.Print Length is a measure of the length of the print area. Stand is theduration in seconds of contact of a paw with the glass plate. StandIndex is a measure for the speed at which the paw loses contact with theglass plate. Swing Speed is the speed (Distance Unit/second) of the pawduring Swing. This parameter is computed from stride length and swingduration. Max Contact at is the time in seconds since the start of therun that a paw makes maximum contact with the glass plate. It can beregarded as the point at which the braking phase turns into thepropulsion phase during Stand. Phase Dispersion Girdle is a parameterabout interlimb coordination using the timed relationships betweenfootfalls of two different paws. All data are presented as means±SEM. *p<0.05 co-crystal of (rac)-tramadol⋅HCl-celecoxib (1:1) vs Tramadol; #p<0.05 co-crystal of (rac)-tramadol⋅HCl-celecoxib (1:1) vs Celecoxib.

Conclusion

Co-crystal of (rac)-tramadol⋅HCl-celecoxib (1:1) produced a superiorbenefit over the single drugs tested in various pain-induced gaitchanges in a rat model of acute monoarthritic pain.

The invention claimed is:
 1. A method of treatment of pain comprisingadministering to a patient in need thereof an effective amount of aco-crystal comprising (rac)-tramadol⋅HCl and celecoxib.
 2. The methodaccording to claim 1, characterized in that the pain to be treated issevere to moderate pain.
 3. The method according to claim 1,characterized in that the molecular ratio between the (rac)-tramadol⋅HCland celecoxib is 1:1.
 4. The method according to claim 1, characterizedin that the molecular ratio between the (rac)-tramadol⋅HCl and celecoxibis 1:1 and that the pain to be treated is severe to moderate pain. 5.The method according to claim 1, comprising administering(rac)-tramadol⋅HCl and celecoxib in a molecular ratio of 1:1,characterized in that it has an orthorhombic unit cell with thefollowing dimensions:a=11.0323(7) Åb=18.1095(12) Åc=17.3206(12) Å.
 6. The method according to claim 1, comprisingadministering (rac)-tramadol⋅HCl and celecoxib in a molecular ratio of1:1, characterized in that it has an orthorhombic unit cell with thefollowing dimensions:a=11.0323(7) Åb=18.1095(12) Åc=17.3206(12) Å; and that the pain to be treated is severe to moderatepain.